Drilling mud is a more or less complex mixture of a base liquid, water or oil, and of various products, which is employed for drilling wells. This mud, injected into the string of drill pipes, moves upwards in the annular space between the walls of the drilled geological formations and the pipe string. One function of the mud is to ensure the continuous removal of the cuttings torn away at the cutting face to prevent the tool from balling up. The cuttings must be conveyed towards the surface in the annular drilling space and must be removed at the surface. To achieve this, the mud must be sufficiently viscous and its bearing capacity must be such that the cuttings can stay in suspension in the mud when the flow rate of the latter is zero. Another of the many functions of the mud is to support the wall of the well so that there are no falls. This function is normally ensured by virtue of the deposition on the walls of a film consisting of the clayey particles present in the mud. Any damage to the walls of the well must be avoided as mach as possible. Other conventional functions of the mud which can also be mentioned are corrosion-free cooling and lubrication of the drilling tool, balancing the pressure in the formation pores, controlling filtration in the formation, etc.
Under normal drilling conditions, when the drilling is sufficiently advanced and when the drilling diameter is smaller than or equal to 30 cm, the physical and physicochemical properties of the mud are strictly controlled and these properties are maintained or are adapted to the drilling conditions by virtue of the addition of various products such as viscosity improvers, weighting materials, filtration reducers, mud thinners (and the like) as various and numerous constituents of the mud.
To fulfil its many functions, the mud must exhibit certain physical and physicochemical characteristics which are adapted to the problems which are encountered and which vary as a function not only of the nature of the terrain encountered but also of the state of progress of the drilling and of the drilling technique employed, and which may be conflicting.
For example, drilling large-diameter wells presents highly specific problems precisely because of their great volume and of the nature of the terrains which are traversed and which are frequently poorly consolidated. These large-diameter drillings are performed in the initial stage of drilling, in order to cross the first 300 to 1500 meters of formation. In drilling of this type priority is generally given to cleaning the hole and to the suspension qualities of the mud. In fact, drilling of this type is characterised by a very large volume of the cuttings to be removed, bearing in mind the surface of the walls, the nature of the terrain and the rate of progress, which is frequently high. The volumes of mud used for drilling large-diameter wells are considerable. To give an example, in the case of a 90-cm diameter well the volume of the drilling hole is of the order of 800 liters per linear meter and the pumps deliver 4,000 to 4,500 liters per minute. The muds employed must therefore be capable of being prepared very quickly and in large quantities.
The drilling of large-diameter wells requiring a high fluid flow rate is usually performed with water by itself, and the insufficiency of the viscosity thereof being compensated by an increase in the pumping rate.
While this technique offers an obvious economic advantage, for example in the case of drilling at sea, its disadvantage is that it results in considerable leaching of the walls. In addition, when such a drilling fluid consisting only of water is employed, it frequently happens that when the movement of the fluid in the well stops, as can happen in the course of drilling, the drilled cuttings settle again at the bottom of the well and result in the drilling tool balling up when drilling recommences.
To increase the bearing capacity of the drilling mud a certain quantity of a reactive clay such as bentonite is sometimes added to the water employed for high flow rate drillings. The use of such a product added to the drilling water is limited, however, because, on the one hand, it greatly increases the cost of drilling, bearing in mind the mud volumes used and the quantities of product which are necessary, of the order of 80 kilograms per cubic meter, and, on the other hand, it requires some hundreds of tons of product to be stored at the drilling site.